In contemporary optical transmission networks, data is transmitted via optical channels with channel data rates of up to 40 Gbit/s. In order to increase the transmission capacity, not only one but a plurality of channels are transmitted on one glass fibre in the wavelength multiplex. From DE 699 04 805 T2, for example, there is known a wavelength multiplexing system which allows the multiplexing and demultiplexing of up to 32 wavelength channels in the preferred wavelength range of 1535-1565 nm (C-band), the channels being separated from one another by 0.4 nm. In contemporary 10 Gbit/s systems, up to 80 channels are transmitted in the C-band in part.
Transmitters for systems of this type, in the form of transmission lasers, are commercially available (as variable frequency) lasers, both with fixed wavelength and with variable wavelengths. The imprinting of data onto the transmission laser by modulation is achieved by means of external modulators after the creation of the optical wave. The data rate is thus determined only by the data rate of the signal supplied to the modulator (client signal) and a constructionally determined maximum data rate of the modulator, and is thus variable. As a rule, however, the data rates are fixed and are not varied. For separating the individual data channels in the receiver, optical filters, which transmit the desired channel and filter out all other channels, are used. The filters may, in accordance with the transmission lasers, either already be fixed to a wavelength during production or be configured so as to be variable. All transmission channels are fixed to a wavelength in the transmitter and maintain said wavelength up until the receiver. A wavelength conversion between the information source and sink does not occur.
The physical parameters of an optical transmission path vary over time. In general, optical transmission systems react to variations in the parameters of the transmission channel more sensitively at higher data rates. Some physical parameters of the transmission medium vary deterministically as a function of the external influences and can be predicted. Other parameters vary stochastically and thus cannot be determined by calculations beforehand. In this case, only the value range of the parameter may be determined, by means of measurements and the tools of stochastics. Deterministic and quasi-static variations can be incorporated into the planning process, in such a way that either a sufficiently great tolerance of the transmission system or tuned compensators are provided. With stochastic and rapid variations, the compensators must accordingly react rapidly and possibly have a large value range or adjustment range available. Both the requirement for fast reaction times and that for wide adjustment ranges lead to a substantial increase of costs in systems of this type. Thus, for example, appropriate PMD compensators are indeed technically feasible for the physical effect of polarization mode dispersion, but the actual use thereof is prevented by the lack of cost efficiency.